This invention relates to the integrated circuit (IC) fabricating processes using photolithographically patterned photoresist materials of the type whose solubility or insolubility is acid-catalyzed during development of the photoresist. More particularly, the present invention relates to a new and improved method of protecting the acid-catalyzed photoresist material from the adverse influence of chemically-basic contaminants, such as ammonia, which are inherently generated by components of the IC as it undergoes fabrication. The present invention permits more precise and resolved etching of the components of the IC during fabrication, which leads to fewer failed ICs due to fabrication defects.
The evolution of integrated circuits (ICs) has involved the continued miniaturization of its components. In addition to reducing the size of the individual components, the spacing or resolution between the various components of the chip has also diminished. The current resolution standard is a sub-micron spacing, in the neighborhood of 0.2 microns. It is expected that future generations of ICs will have even smaller resolutions.
The basic method for forming most of the components of an IC a photolithographic patterning process. A typical photolithographic patterning process involves placing photoresist material on the IC structure and exposing the photoresist to light using a negative or positive mask of the pattern of components. The exposed photoresist material is thereafter developed. The light-exposed photoresist material becomes soluble, which allows it to be washed away. The unexposed photoresist material is undissolved. The soluble areas are removed to provide an opening in the remaining durable mask areas. The open areas define a pattern for the components to be formed, typically by depositing, etching and implanting materials within the exposed areas while the remaining, intact areas shield the other areas. Some types of photoresist materials work in reverse, where the light-exposed areas become the durable mask-like areas and unexposed areas remain soluble and ultimately form the open areas.
In order to obtain the very small resolutions to form the components, the photolithographic patterning process must be capable of patterning and developing the photoresist material with better resolution than the spacing between the finished components of the IC. Photoresist materials which are capable of achieving such high resolutions are very sensitive to the wavelength of the exposure light. A higher degree of resolution in exposure of the photoresist materials requires a shorter wavelength of exposure light. In essence, there is a inverse relationship between the wavelength of the exposure light and the resolution of the developed photoresist materials.
The current generation of photo resist materials respond to wavelengths in the range of approximately 248 nm. It is expected that future generations of photoresist will be capable of securing even higher resolutions and will require exposure light at wavelengths of less than 200 nm. Light sources capable of generating these shorter wavelengths are laser or soft x-ray light sources. The amount of light energy from these sources is significantly low. For example, 248 nm wavelength sources generate in the neighborhood of 5-10 millijoules of energy. By way of comparison, previous types of photoresist material responded to wavelengths in the range of 436 nm, and arc lamp sources which generated those wavelengths were capable of delivering on the order of 100-250 millijoules of energy. The polymeric molecular characteristics of current photoresist materials have been adjusted to obtain good contrast despite the lower energy available from the lower wavelength light sources. Thus, even though exposed to lesser-energy light sources, modern photoresist materials must still obtain significant contrast between the exposed and the unexposed areas. Contrast is responsible for defining the edge characteristics of the features of the components formed on the chip and the resolution between those components.
To respond to shorter wavelengths of lesser power, modern photoresist materials are formed from polymeric chain molecules which contain acid moieties that catalyze to amplify the response initially established by exposure. The initial exposure to light releases some of the acid moieties from the polymeric chains. However, when the exposed photoresist material is thereafter heated, the initially released acid moieties then attack and destroy adjacent polymeric chains and catalyze the release of hundreds or thousands of other acid moieties from adjoining polymeric molecules of the photoresist material. These released acid moieties attack and effectively destroy the polymer chains of the photoresist material in a thorough and complete manner. The broken polymeric chains make the photoresist material soluble so that the open areas can be exposed. The catalyzing effect of the acid moieties accounts for the increased sensitivity and resolution available from modern photoresist material. To preserve the increased sensitivity, it is necessary to assure an effective catalytic response from the acid moieties after the initial light exposure.
It has been recognized that environmental air pollutants can adversely affect the catalytic response of the photoresist. For example, airborne ammonia and ozone are two such contaminants. Ammonia is chemically basic and therefore has the tendency to react with and neutralize the acid moieties of the photoresist material. Neutralized acid moieties can no longer catalyze to destroy the polymer chains within the photoresist material. Neutralizing the acid moieties of the photoresist material results in a diminished resolution capability of the photoresist material, because the photoresist material is not rendered soluble so that it can be eliminated from the open areas.
A variety of techniques that have been proposed to shield the photoresist material from airborne chemically-basic contaminants. One technique is to carbon filter the air that is present over the IC structures during fabrication processing. This technique is somewhat effective against airborne chemically-basic contaminants. Another technique is to add an acid containing polymer material in a layer on top of the photoresist to shield it from the airborne basic contaminants. The acid containing polymer layer neutralizes the chemically-basic contaminants which may diffuse into the photoresist material.
The present invention is founded on the discovery that certain structures involved in the fabrication of modern ICs emit chemically-basic contaminants, such as ammonia, during the course of fabrication. For example, one source of ammonia contaminants is the electrical conductors formed in multiple layers of metal interconnects. Metal interconnect layers are layers of individual electrical connectors which are formed above a substrate of the IC, which route electrical signals to the components of the IC. The metal interconnect layers are vertically separated from one another by a layer of dielectric insulation, and the individual electrical conductors of each interconnect layer are horizontally spaced from one another, also by dielectric insulation. Indeed, the ability to incorporate significant numbers of multiple metal interconnects layers, with each layer having close resolution of the individual conductors, has itself contributed to the evolution and miniaturization of modern ICs.
The metal structure from which the individual conductors of each metal interconnect layers are formed includes an anti-reflective barrier layer. The anti-reflective characteristics of the layer inhibit reflection of the photoresist exposure light from the metal surfaces onto areas of the photoresist material which are not desired to be exposed. The barrier characteristics create an electrically-conductive but chemically-resistant separation of one interconnect level from the next interconnect layer, so that the chemistry of one layer does not interact with the chemistry of another layer and create unacceptable changes in chemical or electrical properties of the interconnect layers. A typical metal layer uses titanium nitride, tungsten nitride or some other metal nitride as an anti-reflectant and as a barrier. Titanium nitride is generated by sputtering titanium into an atomic fog in the presence of ammonia gas. The titanium reacts with the ammonia, stripping the hydrogen ions off the nitrogen and depositing a thin film of titanium nitride. The deposited titanium nitride film is somewhat porous and entraps and contains within it significant amounts of the ammonia from the environment which generated the titanium nitride.
It has been discovered that when the IC structure is heated to catalyze the acid moieties in photoresist material which has been deposited on the titanium nitride, the entrained ammonia is released. The released ammonia enters the layer of photoresist material applied on top of the metal interconnect structure and neutralizes the acid moieties in the photoresist. Neutralizing the acid moieties destroys the ability of the acid moieties to catalyze and decompose the polymers of the photoresist, thereby decreasing the sensitivity of the photoresist and marring the precision of resolution which would otherwise be available from unaffected photoresist material.
The photoresist loses its sensitivity at the location where the ammonia gas has entered the photoresist material and neutralize the acid moieties, which is typically at the interface of the photoresist material to the underlying metal of the interconnect structure layer. Instead of creating a precise photoresist feature, an abnormally shaped or ragged edge structure called a xe2x80x9cfootxe2x80x9d is created. The photoresist at the foot cannot be catalyzed by the acid moieties since those acid moieties at the foot have been chemically neutralized by the diffusing ammonia. The foot becomes an area of insoluble photoresist which inhibits etching of the underlying metal. Since most etching steps are timed to an end point where the desired degree of etching is calculated to have occurred, the reduced etching at the location of the foot creates uneven and ragged structures in the metal structure. For example, when etching a space between adjacent conductors in an interconnect layer, the etch begins in the middle of the space and not at the edge of the metal forming the conductor. Sometime later, the foot is ultimately dissolved by the etchant, but the result of the delayed etching caused by the foot is a nonuniform etch from one side of the space to the other. By the time that the etch step should have been completed, the foot will have caused a xe2x80x9cghostxe2x80x9d metal image to remain at the edges of the space, and this residual ghost metal may short to the adjacent conductor metal line. As little as 10 angstroms of metal between two adjacent metal lines will short them together to such an extent that the IC will not be functional. Moreover, because of the location of the residual metal, it is impossible or very difficult to inspect for this type of defect and discover the source of the problem.
There is no simple way of keeping the ammonia from out-gassing from the titanium nitride layer and still maintain the desired characteristics of the titanium nitride layer. It is possible to heat the titanium nitride layer in an oxygen ambient and convert the surface to titanium oxide which would completely seal the surface and confine the ammonia within the titanium nitride layer. However in later process steps which require access to the titanium nitride, the surface layer of titanium oxide must be removed. Removing the titanium oxide layer requires extra steps which increase the complexity and risk of unreliability of the entire IC fabrication process, thereby increasing the risk of diminished yields of suitably functioning ICs.
The central aspect of the present invention is applying a polymer barrier material to buffer and isolate the surface of the IC structure which contains the chemically-basic material above which the acid-catalyzed photoresist material will be deposited. A barrier is created between the chemically-basic contaminant-containing substance, and the barrier will mechanically and/or chemically stop or inhibit the contaminant from defusing into the photoresist material. Stopping or inhibiting the diffusion of the contaminant stops or inhibits the neutralization of the acid moieties of the photoresist material, allows the acid moieties to catalyze as intended, and generally results in greater precision and resolution in the patterns of developed photoresist and the structures created by the developed patterns of photoresist.
Preferably, the barrier material is a polymer which contains acid moieties. The polymer barrier material is applied in a thin film on the surface of the underlying IC structure which emits the chemically-basic contaminants. The photoresist material is thereafter coated on top of the thin film of the barrier material. The chemically-basic contaminants emitted from the underlying structure are physically and chemically blocked and inhibited by the barrier material. The contaminants that pass into the barrier layer interact with the acid moieties of the barrier material where the contaminants are neutralized. The photoresist material on top of the barrier material remains unaffected, so that exposure to light and heat during development causes the acid moieties to catalyze and destroy the polymer chains as intended to obtain greater precision and resolution in the developed photoresist material. More precise and resolved IC structures are created by using the more precise and resolved developed photoresist material. The effectiveness of the photoresist material becomes independent of the chemical nature of the IC structure upon which that photoresist material is deposited for patterning and development.
The features of the present invention are realized from a method of inhibiting neutralization of acid-catalyzed photoresist from chemically-basic contaminants emitted from a surface of an IC structure above which the photoresist is applied during the fabrication of the IC. The method includes applying a layer of barrier material on the surface of the IC structure from which the chemically-basic contaminants are emitted, applying a layer of the photoresist material on top of the layer of barrier material, and selecting the barrier material to have characteristics to inhibit the interaction of the contaminants with the photoresist. Preferably, the contaminants are inhibited from reaching the photoresist because the barrier layer physically inhibits the movement of the contaminants into contact with the photoresist, or the barrier material contains acid moieties which chemically neutralize the chemically-basic contaminants before they contact the photoresist. In addition, the barrier layer material preferably reacts with the developer which develops the photoresist at approximately the same rate that the developer reacts with the exposed photoresist, to remove the barrier layer from open areas of the developed photoresist at the same time that the photoresist is removed. Further still, the barrier materials are preferably selected to have characteristics which are substantially insoluble in the solvents which are typically present in the photoresist.
Another aspect of the invention relates to a method of increasing spatial resolution available from acid-catalyzed photoresist used in an IC fabrication process by physically inhibiting chemically-basic contaminants from contacting the photoresist placed above a surface of an IC structure which contains those chemically-basic contaminants. Again, the inhibition preferably occurs by chemically neutralizing the contaminants and/or physically inhibiting their movement into contact with the photoresist.
A more complete appreciation of the present invention and its scope may be obtained from the accompanying drawings, which are briefly summarized below, from the following detailed description of a presently preferred embodiment of the invention, and from the appended claims.